1. Field of the Invention
The present invention relates to a collimator having an adjustable focal length, particularly in X-ray inspection systems.
2. Description of the Background Art
Inspection processes using X-rays are used for the detection of critical substances and objects in pieces of luggage or other freight. To this end, multi-stage systems are known whose first stage is based on the absorption of X-rays. For the detection of certain critical substances, such as explosives for example, a second stage is used, with objects from the first stage being selectively delivered thereto. Systems whose operating principle is based on diffraction phenomena are used as the second stage. In this connection, the diffraction angle at which an incident X-ray beam is diffracted depends on the atomic lattice spacing of the material to be inspected as well as on the energy, and thus the wavelength, of the incident radiation. Conclusions can be drawn concerning the lattice spacing and thus the material through analysis of the diffraction phenomena by means of X-ray detectors. Such a two-stage system is disclosed in German patent application 103 30 521.1, for example.
Since X-ray inspection systems operate with extremely low radiation intensities, very sensitive detectors are used. Therefore, to avoid measurement inaccuracies, it is necessary to ensure that only radiation generated by the inspection device strikes the detector. In addition, care must be taken to ensure that only radiation diffracted at a single point is detected, since localization within the object to be inspected is otherwise impossible. Thus, spatial filtering is necessary, which is accomplished by a so- collimator.
Since it is technically very complicated to generate monochromatic X-rays, the sharply defined X-ray beam used for inspection, which is known as a pencil beam, has an energy spectrum that is known from measurements, for example. The result of the Bragg equation is that the incident radiation is diffracted at every point through an angle that depends on the energy of the radiation. Thus, radiation with an energy spectrum is diffracted over an angular range; the diffraction here is rotationally symmetric about the incident pencil beam. In an X-ray inspection, it is desirable to detect only radiation diffracted through a specific angle. This, too, is achieved through the use of a collimator. The transmission range of the collimator corresponds essentially to the surface of a cone whose tip coincides with the point whose diffraction characteristics are to be examined. To examine a region within an object, a large number of points must be focused.
For this purpose, it is known to use a collimator that has multiple parallel apertures with the same aperture angle, and with which it is thus possible to simultaneously focus multiple points on the axis of rotation. However, the use of a non-segmented detector that does not resolve position, and thus provides a common output signal for all focused points, has the disadvantage that the analysis and unambiguous association of the detected radiation to a point of diffraction are difficult. While this disadvantage does not arise when using a segmented detector, which is divided into circular rings that can be analyzed separately, for example, such a detector is complicated and expensive.
Known from German patent application 103 30 521.1, which is incorporated herein by reference, is a method for examining an object space in which the arrangement includes a detector and collimator can be made to travel in the direction of the incident X-ray beam. However, the entire apparatus must have an overall height of more than twice the height of the object to be examined.